i see you have a
Cupra but i had a similar issue last year on my FR. I know you have different brakes but my NSF caliper had seized a bit, this caused the disc to heat up from motorway type driving, to and from work everyday, and actually warp. Not like when people think their discs are warped due to brake pad material build up, the disc had a noticeable "wobble".
When at *cough* 70 *cough* the entire car would vibrate. My theory was the disc would spin, make contact with the seized pad, nudge it, slow the disc ever so slightly, at high speed this was physically noticeable by the vibration. At slow (parking) speed i noticed the car wouldn't free roll as easily as it should, but you had to really think about it to notice it. Threw on a new caliper and discs and it was sorted.